Thesis: Malacath and Boethia are neighbor-twins just like Stendarr and Zenithar. All four are really mirrored reflections of one another.
We learn the concept of divine "neighboring" from Shor son of Shor, an obscure text describing the conflict between the two groups of et'Ada as they led opposing camps of Ehlnofey during the Dawn.
(YMMV as the text is from a forum post by one of the original lorewriters of Morrowind. I don't think that text has ever actually been present in any TES game.)
Note that Shor son of Shor was written from a Nordic perspective so the names of these spirits are different from the names most commonly used for the Divines and Aedra. Shor is Lorkhan. Ald is Auri-El, who is more-or-less also Akatosh.
We see that the greater spirits of the Mundus exhibited recursion and parallelism during the Untime of Dawn along the two fundamental axes of Time (represented by Shor/Lorkhan) and Space (represented by Ald/Auri-El/Lorkhan). We see this vertically down through Time with "Shor son of Shor" and "Shor father of Shor", and we see this horizontally across Space along the Shor/Ald split - see the parallels between "Shor son of Shor" and "Ald son of Ald":
>The Moot looked to the tribe of Ald son of Ald but he would break no oath of the Pact, saying “Shor has paid ransom now three times for the the sins we accused him of, and by that we will hold him as dead and shake not our spears against him or his kin. Of the below he speaks, he is confused by it, for under us is only a prologue, and under that still is only a scribe that hasn’t written anything yet. Shor as always forgets the above, and condemns himself and any other who would believe him into this cycle.” Ald’s shield thane Trinimac shook his head at this, for he was akin to Tsun and did not care much for logic-talk as much as he did only for his own standing. He told his chieftain that these words had been said before and Ald only sighed and said, “Yes, and always they will be ignored. As for the war you crave, bold Trinimac, and all of you assembled, do not worry. A spear will be thrown into this soon, from Shor’s own tribe, and the House of We will be allowed our vengeance.
>Shor found the alcove at the core of the world and spoke to his dead father. He said a prayer to remove any trickery of mirrors and the ghost of Shor father of Shor appeared, saying “Ald and the others have paid time and again for the the sins we accused them of, and by that you should hold them as dead and shake not the spears of your tribe against any of their kind again. Of the above he speaks, Ald is confused by it, for above us is only an ending, and above that still is only a scribe that hasn’t written anything yet. Ald as always forgets the ground below him, and condemns himself and any other who would believe him into this cycle.” But Shor shook his head at this, for he was akin to Ald and did not care much for logic-talk as much as he did only for his own standing. He told his father that these words had been said before and Shor only sighed and said, “Yes, and always they will be ignored. As for the counsel you crave, bold son, and in spite of all your other fathers here with me, that you create every time you spit out your doom, do not worry. You have again beat the drum of war, and perhaps this time you will win.” Shor son of Shor returned then to us on the mountaintop.
But we also see within the respective camps, as evidenced by Shor's shield-thanes Tsun and Stuhn:
>Tsun took her by the hair, for he was angered by her words and heavy with lust. He was a berserker despite his high station, and love followed battle to his kind. “You weren’t made for that kind of thinking,” Stuhn said, dragging Dibella towards a whaleskin tent, “Jhunal was. And no one should be speaking to him now.” Tsun eyed the Clever Man who had heard him. “Logic is dangerous in these days, in this place. To live in Skyrim is to change your mind ten times a day lest it freeze to death. And we can have none of that now.
>Kyne could have stopped all of this but did nothing but stare at the crowd of Nords around her. Stuhn and Tsun were shifting and it was still uncouth to prevent this kind of neighboring.
Tsun and Stuhn are Stendarr and Zenithar, Divines in the pantheon of Man, the descendants-cum-inheritors of Shor/Lorkhan's camp. But we also see a third party "neighbor" these twins: Trinimac. He is the Merethic parallel to Tsun, the shield thane of Ald, who is depicted as parallel to Tsun with wording identical to the parallel between Shor and Ald:
>Ald’s shield thane Trinimac shook his head at this, for he was akin to Tsun and did not care much for logic-talk as much as he did only for his own standing.
Trinimac also shifts with Tsun just as Stuhn does:
>He didn’t need to explain what he had learned, for we had been there with him. Trinimac left Dibella in his tent as we assembled, and he had not touched her, frozen in the manner of the Nords when we are unsure of our true place, and asked his brother to rearm him. Stuhn was confused for a moment, thinking this an odd shift ...
We also see a continuing inverse relationship between Zenithar and Malacath in the ESO quest "Z'en and Mauloch":
>>"Dalaneth told stories of these journals, and now you bring them to me. Hm. Since the earliest days, blood and toil have known no end in our hearts. Mauloch demands a blood price. Z'en's price is no different."
>Why do you say that?
>>"Our gods both demand payment in kind. Of course, that means each demands retribution against the other. In the valley, it has always been paid in blood."
>Do you think you'll ever stop fighting?
>>"Every tale has an end. Every end allows a beginning to set hold. It is a glance, tenuous though it seems to be. Z'en's presence fades from this world as Mauloch grows stronger. Perhaps these journals will give comfort to those who fear the end."
And yet, unlike Tsun and Stuhn (Stendarr and Zenithar), there is only one Trinimac in Ald's camp.
Or is there?
Historically most of the lorebooks agreed that the Aedric champion Trinimac, the god who cut out Lorkhan's heart atop Adamantine Tower at Convention on Auri-El's orders, was publicly mocked or impersonated and somehow consumed by Boethia, who metaphysically tortured and apparently "shat" him out as Malacath. See The Changed Ones, The Fall of Trinimac, and The True Nature of Orcs (which was banned by the Daggerfall Covenant as anti-Orc propaganda during the events of ESO).
But there are two other versions of this story.
In Mauloch, Orc-Father, Malacath was Trinimac and Boethia an imposter; Trinimac the Aedra ttransformed himself into a Daedra by "tearing the shame from his spirit."
And in From Exile to Exodus, Boethia was Trinimac and Malacath an imposter; Trinimac the Aedra transformed herself into a Daedra by "the burden of rending divinity from the one she loved."
I am reminded of Sermon 11
>The ruling king is armored head to toe in brilliant flame. He is redeemed by each act he undertakes. His death is only a diagram back to the waking world. He sleeps the second way. The Sharmat is his double, and therefore you wonder if you rule nothing. Hortator and Sharmat, one and one, eleven, an inelegant number. Which of the ones is the more important? Could you ever tell if they switched places? I can and that is why you will need me.
We see in the broad thrust of TES metaphysics a fractal and interweaving progression from One to Many. The Dreamer becomes Anu and Padomay, who fight over Nirni. These two individuals give rise to Ald son of Ald and Shor son of Shor and their respective clans - but Shor is a parallel of Ald and Ald of Shor. They lead the wandering Ehlnofey and the old (Ald) Ehlnofey. Auri-El is the son of Ald and Ald's shield thane (and Auri-El's right hand) is Trinimac; Lorkhan is the son of Shor and Shor's shield thanes are Stuhn/Stendarr and Tsun/Zenithar; they are parallels to one another. So Trinimac loves Auri-El and sunders Lorkhan by tearing out his heart; and Trinimac is exposed as a hypocrite who believes that weeping "tears were the best response to the Sundering" and tears out his shame from his own chest.
Trinimac hates Lorkhan and Trinimac loves Lorkhan. Malacath and Boethia. And Lorkhan lead the Aedra, including Auri-El, to become entrapped in the Mundus and was slain by Auri-El for it; Stendarr and Zenithar - Divines - are within the Mundus but Boethia and Malacath - Daedra - are not.
There's a balance, here, I think.